1. Field of the Invention
The present invention relates to a servomotor control system and, more particularly, to a servomotor control system that enables high-speed oscillating motion to be highly precise.
2. Description of the Related Art
As for jig grinders and the like, there is a need for a control system that enables highly precise machining during high-speed oscillating motion of a grinding stone. Particularly, highly precise upper dead centers and lower dead centers in vertical oscillating operations are required.
A jig grinder is a machine tool that carries out finish grinding for sides of jigs, trimming dies, gages, and other tools for machines and for internal surfaces of holes. As shown in FIG. 13, a jig grinder grinds the side 4 of a workpiece 2 placed on a table (not shown) with the outer circumference of a cylindrical grinding stone 6; the cylindrical grinding stone 6 rotates on its axis, that is, a rotation axis 7 while being vertically oscillated in the axial direction of the rotation axis 7.
With a conventional control system used for a jig grinder, precision is increased by correcting commands to suppress tracking delays in servo control. Specifically, to increase the precision of the upper dead center and lower dead center, an oscillation command is compared with the operation of the servo, and a commanded amplitude is gradually increased (see Japanese Patent Application Laid-Open No. 4-289903 and Japanese Patent Application Laid-Open No. 3-31906).
In this method, however, a long tracking time is taken to obtain desired precision. There is another problem with stability; a divergence may occur due to a change in a machining condition such as a change in amplitude.
A known method that achieves highly precise tracking at high speed in response to a repetition command is learning control (repetitive control). Two types of learning control are used; a time synchronization method in which learning is performed with respect to time (see Japanese Patent Application Laid-Open No. 3-175502) and an angle synchronization method in which learning is performed with respect to angle (see Japanese Patent Application Laid-Open No. 2004-280772).
In the learning control based on the time synchronization method, described above, a delay memory the length of which is equal to the length of a commanded repetitive cycle is provided in a control unit, so this type of leaning control cannot be used when the commanded repetitive cycle dynamically changes. In the learning control based on the angle synchronization method, described above, a reference angle that monotonously increases in synchronization with a commanded repetitive operation is needed, so this type of learning control cannot be used when no reference angle is given.
In high-speed oscillating motion performed by, for example, a jig grinder, a commanded repetitive cycle dynamically changes according to the machining condition. There is no reference angle that should be in synchronization with a commanded repetitive operation, making it impossible to use a reference angle in servo control. Accordingly, neither the learning control based on the time synchronization method nor the learning control based on the angle synchronization method can be used.